It has been known to provide security documents with a foil element comprising a security feature formed by a diffractive structure, such as a surface or volume hologram or grating. Such foils cannot be forged by conventional printing or copying techniques, thereby rendering the documents more secure.
However, such documents may be tampered with if it is possible to remove the foil element without damaging it. Even though a removal of the foil element can be detected by verification devices, such devices need to be equipped with dedicated sensors for sensing the presence of the foil element, which renders such equipment more expensive.